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1
Pointing and Stabilization of Lightweight Balloon Borne Telescopes
presented at the
SwRI LCANS 09 Balloon Workshop onBridging the Gap To Space
Lightweight Science Payloads on High-Altitude Long-Duration Balloons and Airships
26 October 2009
Larry GermannLeft Hand Design Corporation
2
The Purpose of a Precision Pointing System
• Perform line-of-sight stabilization
– Correct atmospheric turbulence
– Correct vehicle base motion
– Correct vibration of optical elements
– Correct force or torque disturbances
– Correct friction-induced pointing errors
• Perform scanning function to extend the Field of Regard beyond the telescope’s Field of View
• Perform chopping function
• Perform dither function
• Quickly slew and stare among a field of targets
3
When a Precision Pointing System is Needed
• When the required pointing stability cannot be achieved by the platform attitude control system
• When the field-of-regard requirement is larger than the instrument’s achievable field-of-view
• When chopping is required to calibrate the optical sensor
4
Precision Pointing Systems Cover Large Ranges of
Precision and Field-of-Regard
• Fields-of-Regard from 100 microradian to continuous rotation are considered.
• Precision is defined as positioning resolution, stability and following accuracy.
1
10
100
1000
10000
0.001 0.01 0.1 1 10 100
Fie
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f R
egar
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ans)
System Precision (micro-radians)
Mass-Stabilized Telescope Satellite, like HST
Fine-Steering Mechanism (FSM) with a Coarse Steering Mechanism
Coarse-Steering Mechanism
Single Full-ApertureFlexure-Mounted Steering Mirror
Single Full- or Reduced-ApertureFlexure-Mounted Steering Mirror
Full-Aperture FSM Sensor Noise LimitF
SM
Sen
sor
Noi
se L
imit
wit
h 10
x O
ptic
al G
ain
Fri
ctio
n L
imit
FSM Sensor Dynamic Range Limit
Increasing Cost
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Line-Of-Sight Stabilization, Stability Correction Ratio
Correction Ratio Amplitude (f) = Base Motion (f) / Residual LOS Jitter Requirement (f)
Pointing System Cost is Related to the Correction Ratio Spectrum
6
Dominant Sources of Vehicle Base Motion
• LEO Spacecraft
– Thermal Shock from Transitions into & from Umbra
– Attitude Control System (ACS) exciting vehicle bending modes
– Solar Array Drives
• High-Altitude Lighter-Than-Air
– ACS exciting pendulum & suspension cable bending modes
– Payload Mechanisms
– Station-Keeping Propulsion, if applicable
• High-Altitude Heavier-Than-Air
– Air Turbulence exciting vehicle bending modes
– Propulsion
7
Typical Precision Pointing System Components
• The components of a typical precision pointing system include:
– Beam-expander telescope
– Fine-steering mechanism or fast-steering mechanism: two-axis reduced-aperture, full-aperture steering mirror or isolation system
– Coarse-pointing mechanism: vehicle attitude control system, two-axis gimbaled telescope or full-aperture steering mirror
• Payload motion sensor suite: inertially or optically referenced
• In general, both fine-and course-pointing mechanisms are required when system dynamic range >10^5 @1kHz or >10^6 @10Hz is required, exceptions include a mass-stabilized satellite ACS for the single pointing stage
• Flexure-mounted fine-steering mechanism is required when system following accuracy requirement exceeds friction- or hysteresis-induced limits
8
Fine- and Coarse- Pointing Mechanisms
• Coarse-Pointing Mechanism
– Performs large-angle motions
– Can be vehicle ACS or a bearing-mounted mechanism
– Keeps FPM near the center of its travel range
• Fine-Pointing Mechanism
– Performs high-frequency portions of pointing motions
– Performs high-acceleration motions
– Accurately follows commands
– Corrects or rejects base motion and force and torque disturbances
– Can be reaction-compensated (a.k.a. momentum compensated)
9
2-Axis Fast-Steering Mechanism Technology is Mature
• Apertures for beam sizes from 15mm to 300mm are available, 116 x 87mm for a 75mm beam shown
• -3dB closed-loop servo control bandwidth up to 5,000 Hz
• Range of travel up to +-175mrad (+-10degrees) is available
• A variety of mirror substrate materials are proven– Aluminum– Beryllium (shown here)– Silicon Carbide– Silicon Carbide Foam– Zerodur– BK-7
10
CE50-35-CV-RC2 FSMIs Simple, Robust and Mature
•The CE75-35-BK SN140
•BK-7 mirror
•76.2mm diameter aperture
•+-35mRad travel
•120 Rad/Sec2/rootW efficiency
•2,300 Rad/Sec2 acceleration
•wave PV @633nm surface figure error
•450 Hz -3dB closed-loop servo control bandwidth
11
CE75-35-ZD Represents LHDC’s line of Cost-Effective FSM
•CE75-35-ZD SN147, Zerodur mirror
•76.2mm diameter aperture
•+-26mRad travel
•A custom abbreviated frame
•9,000 Rad/Sec2 acceleration
•120 Rad/Sec2/rootW efficiency
•0.165 wave PV @633nm surface figure error
•250 Hz -3dB closed-loop servo control bandwidth
•Coating is highly reflective at 1.5um
12
FO50-175-ALHas Space-Flight Experience
•FO50-175-AL SN106
•Aluminum mirror
•80.7 x 60mm polished aperture
•+-175mrad travel
•380 Hz -3dB closed-loop servo control bandwidth
•7,000 Rad/Sec2 acceleration
•Proven in low-earth orbit
13
FO50-35-SC-RT7 Achieves Record Servo Control Bandwidth
•FO50-35-SC-RT7 SN133
•Silicon carbide mirror
•80.7 x 60mm polished aperture
•+-5mrad travel with the reduced-travel option
•5,000 Hz -3dB closed-loop servo control bandwidth when base-referenced
•6,000 Hz -3dB closed-loop servo control bandwidth when optically referenced
•3,300 Rad/Sec2 acceleration
14
The Fine-Steering Mechanism Can Be An Active Isolation System
Non-Contacting 6-DOF Active Isolation System
• Non-Contacting electromagnetic actuators
• Non-Contacting sensors
• Highly flexible umbilical transfers signals with <0.1 Hz suspension resonant frequency
– minimal transfer of base motion forces
• Accelerometer- and position-referenced stabilization servos
• IS2-10 Isolation System
– Occupies a 25mm thick disk
– ±2mm travel in 3 axes
• IS5-40 Isolation System used here as a base-motion simulator
– ±5mm travel in 3 axes
15
Servo Functional Block Diagram
16
Flight-Format Servo Control Electronics is Available
• SC03-BD
• 2 Channels Servo Control
– Position-Referenced Loops
– Current-Referenced Drivers
– Optical Tracking Reference
– Position Sensor Reference
• Light Weight
– 150 Grams
• Full Military Temperature
• Up to +-45V, 10A Driver Capability
17
Servo Control Electronics Available in a VME-6U
Single-Card Format
SC02-BDSingle-Card VME-6U Format
Contains All Servo Functions- Pointing and Tracking Modes- Current-Referenced Driver- High-Temperature Driver Shutdown
18
Components of Pointing Accuracy
• Fine- and course-steering mechanism pointing accuracy is defined in several ways:
– Positioning resolution and position reporting resolution
– Line-of-sight jitter and position reporting noise
– Short-term positioning drift and position reporting drift
– Long-term positioning drift and position reporting drift
– Positioning thermal sensitivity and position reporting thermal sensitivity
– Positioning linearity and position reporting linearity
19
Imaging Resolution Limit isRelated to Altitude and Aperture
• Imaging resolution is constrained by the optical diffraction limit, which is a function of altitude and telescope aperture
• Image resolution is defined as a distance on the ground from 30km altitude
20
Positioning and Reporting Linearity
• Positioning linearity is defined as the difference between commanded and achieved position over the operating ranges of travel and temperature
– Dominated by friction, disturbances and position sensor error
– Position sensor error is dominated by thermal sensitivity
– Typically not much better than 0.04% of travel
• Reporting linearity is the difference between reported and achieved position over the operating ranges of travel and temperature
– Dominated by position sensor error
21
Fast Beam Steering is Defined as Servo Control Bandwidth
• Fast beam steering is defined as the ability to follow a small-amplitude sine wave at various frequencies
• Generally defined as the frequency at which the closed-loop servo response falls by 3dB
• Alternately defined as the 0dB open-loop frequency
22
Fast Beam Steering is alsoDefined as Acceleration Capability
• Fast Beam Steering is sometimes defined as the highest frequency at which the mechanism can perform a full travel sine wave
• This is limited by the mechanism’s acceleration capability
• Acceleration is shown here in terms of peak and continuous capability
23
Non-Linear Characteristics Limit Positioning Accuracy
• Friction-induced pointing error
– Typically associated with ball or sleeve bearings
– Peaks at turn-around condition (stick-slip)
– Friction-induced error amplitude can be readily estimated
• Peak Pointing Error ~ 2 * Friction Torque / Inertia / Bandwidth2
• Hysteresis-induced pointing error
– Typically associated with ceramic actuators
– Typically quantified in terms of % of travel range
– Effect are similar to friction effects
24
Precision Pointing Systems Offer Many Benefits
• Extended Dynamic Range, – Up to 9 orders of magnitude– Up to +-180 degree Field of Regard– As low as nanoradian line-of-sight stability
• High servo control bandwidth, up to 5,000 Hz– Correct disturbances up to 1,000 Hz
• Stable Line-of-Sight– Correct for platform vibrations– Correct for aero turbulence
• Agile Beam-Steering for scanning, chopping, dither, etc.– Up to 15,000 rad/sec2 acceleration– Up to 30 rad/sec rate
25
Many Precision Pointing Instrumentsare Suitable for
Near-Space Platforms
• LIDAR measurements of forest canopy
• LIDAR measurements of foliage, carbon stock under canopy
• LIDAR measurements of targets under foliage or camouflage
• LIDAR topology measurements under foliage
• 0.1m resolution over a 20km circle on ground from 100km altitude
• 0.03m resolution over a 6km circle on ground from 30km altitude